This invention describes improvements to differential pressure sensors/transmitters utilizing an isolation (or protection) diaphragm and fill fluid to transmit a process pressure to the semiconductor pressure sensors.
Pressure is a fundamental process variable that can be used to determine flow rates, fluid levels, altitude, and even temperature. Sensors and transmitters are used in the field to monitor and measure any number of industrial processes. They are employed in oil and gas, pulp and paper, pharmaceuticals, food and beverage; any industry where liquids, gases, or slurries are stored, conveyed or otherwise processed. In the field, with differential pressure applications for industrial processes, sensors/transmitters may experience large line pressures, occasional overload pressures or extreme swings in operating temperatures; of which many may be fatal to the process sensor or transmitter.
With industrial pressure sensors, most of the design effort is in protecting the sensor element and thereby protecting the process from damage due to a control system failure. In such sensors, the sensing element produces an electrical output signal proportional to a pressure differential, which is determined by detecting the differential between a high, and a low-pressure port in the sensor mechanism. With industrial sensors, a structure is designed and provided to protect the sensor from circumstances that can degrade its performance, such as corrosion, electrical shorts, and over pressure.
In sensors of the type this invention is designed to improve, the sensor is provided within a body that is divided into sections that seal the sensor from damage and that provide a means to determine the differential in the pressure measurements—including high and low pressure points that are in communication with a process fluid. The static pressure of each of the pressure points provides the sensor with a measure of differential pressure—the larger the difference in the static pressures from the high and low pressure points indicate higher differential pressure, the closer the two points are indicates lesser differential pressure. The primary protection is the isolation diaphragm. The isolation diaphragm is the interface between the pressure sensor system and the process fluid. It protects the sensor from corrosive process environments. It is compliant allowing pressure to transfer from the process fluid to a fill fluid. The fill fluid communicates the pressure to the sensing element, and acts as a dielectric to prevent electrical leakage.
Protecting from overpressure is normally only done with differential pressure sensors, because they have to be sensitive enough to measure differential pressures that may be several orders of magnitude less than the process system's pressure, or the “line pressure” of the system. If the sensing element were exposed to a differential pressure equal to line pressure, the element could be destroyed. Overload protection functions by stopping fluid flow when a design differential pressure threshold is reached. The diaphragm design used in the prior art, for controlling the overload engagement, is a corrugated diaphragm or bellows. Corrugation gives the diaphragm strength and resilience, however, using corrugated diaphragms complicates the design and results in the need to add extra, non-working, fill fluid to the system to fill volumes in the system that occurs due to the inherent shape of such. This extra fill fluid becomes problematic as sensor measurement requirements become more precise and critical. The fill fluid, while typically more ideal than the process fluid, still undergoes physical change due to pressure and temperature.
Additionally, high line pressure processes can substantially affect the output of the sensing element. A common method for compensation is to include a line pressure sensor to provide a signal that can be used for adjusting the output of the differential signal accordingly. However, temperature can compromise the performance of overload protection. Another issue is that the overload protection system is sensitive to fluid volumes: the relatively large coefficient of thermal expansion of the fill fluid with respect to the system's body will change the engagement pressure of the overload protection scheme. To mitigate these effects it has been a goal of the manufacturers of sensors to keep the fill fluid volumes to a minimum. The desire to keep the volumes low, however, has in the past, been thwarted by the prior art's use of corrugated diaphragms, as noted above. In addition, the space requirements within the sensor casing is also determinative of the amount of volume, that has until now, needed to be filled with fluid.
Prior issued patent and published applications are key to an understanding of the state of the art of the present invention and show steps towards the mitigation of problems in prior sensor technology; the teachings however are insufficient to truly overcome the problems that to date plague pressure sensing equipment. U.S. Pat. No. 7,454,975 to Louwagie et al. describes using solid material to decrease fill fluid volume. However, the inventors continue the use of corrugated diaphragm thereby continuing the complexity of the housing and the need for more fill fluid to compensate for the shape of the diaphragm. U.S. Pat. No. 5,531,120 to Nagasu et al. describes the use of 3-diaphragm protection strategy, but continues the outdated use of a concentrically corrugate actuation diaphragm; Nagasu et al further teaches no passive thermal expansion compensation. US published patent application 2004/0040384 of Kurosawa et al. teaches a 4-diaphragm protection mode which is dissimilar to the present invention, but begins to described an attempt to address the issue with small concentrically corrugated actuation diaphragms. The device of Kurisawa et al. has two actuation diaphragms one for the low pressure side and one for the high pressure side so that in theory they can get equal overpressure thresholds for both sides.
In addition to the different uses of materials, types of diaphragms and fill space, prior attempts to overcome the problems of such devices have also centered on the sensor, types of sensors and location of sensors within pressure sensor devices. U.S. Pat. No. 4,841,776 to Kawachi et al. discloses a differential pressure sensor with a static pressure sensor but differs from the present invention in that the static pressure sensor is in a separate chamber to protect that device. U.S. Pat. No. 4,909,083 to Fazeli et al. discloses a differential pressure sensor with a static pressure sensor; however the two sensors are mounted on separate pedestals that are separate from the glass to metal seal. U.S. Pat. No. 5,259,248 to Ugai et al. discloses a differential pressure sensor with a static pressure sensor that uses an integrated sensor; that is it uses a sensor that detects both static and differential pressure. U.S. Pat. No. 5,029,479 to Bryan shows the use of a differential pressure sensor with a static pressure sensor that uses an integrated stacked MEMS sensor assembly.
In addition, U.S. Pat. No. 4,329,877 to Hershey, attempted to overcome the problems surrounding the volume of fill fluid by adjusting the stiffness of the actuation diaphragm rather than adjusting the volume behind the isolation diaphragm. U.S. Pat. No. 4,135,408 to Di Giovanni makes note of the issue of fluid volume problems due to manufacturing issues but suggests using available space to insert slugs before the device is sealed. Di Giovanni does not address the issue of adjusting the overload after the sensor is sealed.
It is therefore an object of the present invention to provide a means for more accurately and efficiently detecting the differential pressures in industrial applications while providing a more accurate, more efficiently constructed and better protected pressure protection device than in the prior art.
Other objects and advantages of the present invention will become apparent as the description proceeds.